While the present invention relates to the formation of any metal by treating molten iron with a reactant, the formation of one type of metal, ductile iron, is of particular concern.
Ductility is generally the capability of a material to bend, warp, or to otherwise plastically deform without failure. Ductile irons generally exhibit a relatively high yield strength which is superior to the yield strength of both grey and malleable irons. Generally, a molten base metal iron may be transformed into ductile iron by inoculating the base metal with a suitable nodularizing agent to form graphite spheroids in the base metal. A high percentage of graphite spheroids generally results in satisfactory ductile iron. Examples of suitable nodularizing agents for producing iron with speroidal graphite are magnesium, calcium, potassium, lithium, sodium, and beryllium. The most commonly used nodularizing agent is magnesium.
One prior method for the production of iron with spheroidal graphite is the "pour-over" process. The pour-over process utilizes a treatment ladle into which a magnesium containing alloy is placed. Then, the molten iron is poured into the treatment ladle to cause vaporization of the magnesium. As the magnesium vaporizes, it is released into the molten iron and forms spheroidal graphite nodules. While in practice it is theoretically possible to use pure magnesium as the nodularizing agent, due to the violence of the reaction between the molten iron and the pure magnesium, most production processes utilize a magnesium containing alloy which moderates the reaction by reducing the rate at which the magnesium vapors are released into the base iron. For example, the percentage of magnesium in the nodularizing agent may vary between 3 percent and 9 percent.
A variation of the pour-over process utilizes a ladle having a teapot spout communicating with the bottom end of the ladle. The teapot spout ladle also includes a dividing wall extending upwardly from the bottom of the ladle and forming a compartment within the ladle for containing the magnesium containing alloy or other nodularizing agent. The dividing wall must reach higher than the highest point of the entrance of the spout into the ladle so that when the molten iron is poured into the teapot spout, the ladle fills from the bottom up, and there is little turbulence when the magnesium containing compartment is flooded.
With this teapot ladle process, it has in the past been assumed that gases, including magnesium fumes, must be allowed to escape from the ladle during the reaction in order to prevent a pressure build-up within the ladle. It has been assumed that such a pressure build-up would cause the molten iron to be blown back out of the teapot spout or cause the ladle to explode. Therefore, although the ladle has been covered on top with a cast iron plate to reduce fuming and to prevent iron from splashing out of the ladle, enough play has been allowed between the top of the ladle and the cast iron plate to equalize the interior pressure build-up with the outside atmosphere. During the reaction, only gases were permitted to escape.
Typically, the cast iron plate has a hole therein positioned above the magnesium containing compartment so that the magnesium containing alloy can be easily placed in the compartment, and a cover is slid over the opening during the reaction.
Disadvantages of this process utilizing a ladle with a teapot spout are that a small amount of magnesium fumes escape during the reaction, and that heat is lost. A further disadvantage is that a significant portion of the magnesium is lost in the form of magnesium oxide or magnesium sulfide. Since magnesium containing alloys are relatively expensive, it would be desirable to increase the magnesium recovery of the process.
Attention is directed to the following U.S. patents which relate to the field of the present invention:
______________________________________ Patentee U.S. Pat. No. Issued ______________________________________ Windish 4,391,636 July 5, 1983 Mannion 4,312,668 January 26, 1982 McPherson 4,210,195 July 1, 1980 Roberts 4,134,757 January 16, 1979 Cole 4,033,766 July 5, 1977 Alt 3,955,974 May 11, 1976 Lee 3,870,512 March 11, 1975 Kusaka 3,833,361 September 3, 1974 McCaulay 3,819,365 June 25, 1974 Anders 3,802,680 April 9, 1974 Mantell 3,650,516 March 21, 1972 Parlee 3,619,173 November 9, 1971 ______________________________________